Continuous ratchet drive assemblies are known in the art and used for many applications in tools and machinery. A continuous ratchet drive assembly is characterized by the capability of the gear assembly to receive two input torque forces applied from two different directions and to translate the bi-directional input forces to an output force in a single direction.
There are many situations in which it is desirable for a dual input torque to produce a single output torque because either a human hand or a gear cannot accommodate 180-degree rotation. Continuous ratchet motion is intended to prevent the loss of half the input torque motion.
During a surgical procedure, it is desirable to accommodate the range of motion of a surgeon's hand and to use force effectively, by applying it to the instrument in order to avoid fatigue. However, it has not been possible to apply mechanical principals used for tools and other continuous ratchet drive assemblies to medical instruments.
This is primarily because the instruments known in the prior art rely on entraining mechanisms which engage and slide mechanical components either over or past each other. Entraining mechanisms involve contact between two or more metal components and require lubrication. However, surgical instruments cannot use lubricants. Tools known in the prior art rely upon parts that generally require the use of lubricants or non-medical grade coatings. Additionally, the movement of these assemblies is not precise or stable enough to withstand medical procedures and subsequent sterilization.
Without lubrication, medical instruments are prone to galling. Galling is a form of wear characterized by localized material transfer, removal, or formation of protrusions when two solid surfaces slide against each other.
It is a problem known in the art that medical instruments must be non-galling and capable of functioning without lubricants.
It is also a problem known in the art that all components and assemblies within a medical instrument must be safe, stable, and capable of high precision and smooth movement.
For example, U.S. Pat. No. 5,931,062 (Marcovici '062) discloses a continuous ratchet drive gear assembly having a reversing mechanism coupling two driving elements. This coupling forces the two ratchet gears together forcing them to always rotate in opposite directions so that one driving element entrains the shaft and the other driving element overruns the shaft. This causes the shaft to always turn in only one direction, regardless of the direction of rotation of the driving elements. The apparatus taught in Marcovici '062 cannot be used in medical instruments because it requires entraining.
The entraining mechanism taught by Marcovici '062 involves substantial sliding contact between two metal elements requiring use of lubrication.
For example, U.S. Pat. No. 5,176,038 (Inokuchi I '038) and U.S. Pat. No. 5,259,259 (Inokuchi II '259) disclose a mechanism to convert the linearly reciprocating motion of a radial handle into a unidirectional rotation of an output shaft through a racks and pinion combination utilizing toothed one-way clutches capable of selective deactivation to reverse output shaft rotation. During operation, the pinions slide in contact with a stationary plate, requiring application of lubricant between the metal surfaces that will be in contact. Similarly, the drive gears situated on shafts have metal surfaces that slide in contact with the bottom surface of the sun gear during rotation, which likewise requires application of a lubricant between the metal surfaces of the drive gears and the sun gear to prevent galling and corrosion.
In another example, European patent 2,586,570 A1 (Wang '570) discloses a manual tool having a bidirectional mechanical converting means. In one embodiment, a bidirectional mechanical conversion scheme with a reversing means comprising a reversing element sleeved on the main shaft with openings through which pawls can engage with the toothed inner surface to form a one-way clutch and an elastic element between each pair of pawls to keep the pawls diverging against the toothed inner surface. However, the use of an elastic element cannot be autoclaved and thus precludes use of the reversing mechanism for medical use. Further, an elastic element will not provide the required smooth rotational motion required for use in surgical procedures.
It is desirable to adapt this concept to a medical tool, but there are many engineering issues to overcome, including the use of lubricants and the need to prevent galling.